1. Field of the Invention
The invention relates to a control device and control method for a belt-type continuously variable transmission that is mounted on a vehicle equipped with four-wheel drive.
2. Description of the Related Art
In recent years, vehicle transmissions increasingly employ continuously variable transmissions (CVT), which are able to steplessly change speed ratios. Currently, the predominant vehicle-mounted continuously variable transmission is a belt-type continuously variable transmission that is formed of a combination of a pair of variable-diameter pulleys and a belt wound around those pulleys. The belt-type continuously variable transmission changes the ratio between belt winding radii of the two input-side and output-side pulleys (pulley ratio) to shift speeds.
In the thus configured belt-type continuously variable transmission, the belt transmits torque while slightly slipping on the pulleys. Motion of the belt causes a slip due to μ-V (friction coefficient-speed) characteristics, so energy that vibrates the belt is constantly generated when the continuously variable transmission is in operation. On the other hand, the belt wound around the two pulleys has free belt spans that are not in contact with either pulley (note that portions wound on the pulleys form a circular arc shape, and portions between the pulleys, forming a substantially straight shape, are termed belt spans). Thus, belt span vibrations occur in the belt spans during operation of the continuously variable transmission.
Japanese Patent Application Publication No. 2001-108082 (JP-A-2001-108082) describes an invention that focuses on such belt span vibrations in the belt-type continuously variable transmission. In the invention described in JP-A-2001-108082, belt slip due to insufficient belt clamping pressure is detected if the vibrations of the belt spans increase, and, when it is detected, belt clamping pressures of the pulleys are increased to suppress belt slip.
The frequency of the above described vibrations of the belt spans may coincide with the natural frequency of certain vehicle components to cause resonance. This may generate muffled noise that makes a vehicle occupant feel uncomfortable. The above muffled noise is particularly remarkable in four-wheel drive vehicles as described below.
Certain four-wheel drive vehicles are able to change the torque distribution ratio between the front and rear wheels depending on a situation. Some of these four-wheel drive vehicles may also switch between two-wheel drive and four-wheel drive depending on a situation. In two-wheel drive, the torque is distributed only to main driving wheels (for example, front wheels). In four-wheel drive, torque is distributed to both the main driving wheels and auxiliary driving wheels (for example, rear wheels). In these four-wheel drive vehicles, the auxiliary driving wheels are drivably connected to the transmission in four-wheel drive, and the auxiliary driving wheels are decoupled from the transmission in two-wheel drive. Thus, when switching from two-wheel drive to four-wheel drive, vibrations of the belt spans in the belt-type continuously variable transmission propagate over a wider range. Thus, in four-wheel drive, belt span vibrations propagate also to components that rarely experience belt span vibrations in two-wheel drive. Hence, even if no component resonates with belt span vibrations of the belt spans during the two-wheel drive, resonance may occur as the belt span vibrations propagate over a wider range during four-wheel drive.
To avoid the above resonance phenomenon, it is necessary to design components, provided within the range over which belt span vibrations propagate, so that the natural frequency of each component does not coincide with the frequency of the belt span vibrations. However, it is difficult to design all the components located within the range over which belt span vibrations propagate. Moreover, it is further difficult to apply the above design to all the components located within the wider range over which belt span vibrations propagate during the four-wheel drive.
Note that most of four-wheel drive vehicles that are able to switch between two-wheel drive and four-wheel drive use a two-wheel drive vehicle as a base for development. Here, even when the base two-wheel drive vehicle is designed so as not to resonate with the vibrations of the belt spans in the belt-type continuously variable transmission, some components may resonate with the belt span vibrations when switched to the four-wheel drive. In this case, to eliminate components that cause resonance, it is necessary to largely change the design from the base vehicle when the four-wheel drive function is added. Thus, actually, prevention of muffled noise during the four-wheel drive requires a compromise to some extent.
In addition, even in full-time four-wheel drive vehicles that regularly run in four-wheel drive as well but varies torque distributed between the front and rear wheels, belt span vibrations may propagate to various portions of the vehicle varies as the torque distribution changes. For example, even if a component that hardly receives the influence of the belt span vibrations when torque distribution ratio of the rear wheels is small, the component may receive non-negligible influence from the belt span vibrations as the torque distribution ratio of the rear wheels increases. Thus, in full-time four-wheel drive vehicles that vary torque distribution between the front and rear wheels, the same problem may arise.